I would have to say the Chase Freedom Plus, hands-down. Must be the PLUS though to really get the benefits of the card. I earned the $250 cashback check after only 5 months. So I figure in a year I'll get back $500 in cash back, that makes the $30 membership fee well worth it. PenFed would be great if you travel by car nonstop and can rack up the money in 5% back on gas, otherwise 2% on groceries and 1.25% on other things doesn't compare to what I can do with the Freedom PLUS.
Discover More has been a great one, too. Particularly if you shop online their rewards for online shopping are FANTASTIC. Can't beat 'em.